Induction resistivity well logging devices are known in the art for measuring the electrical resistivity of earth formations penetrated by wellbores. Electrical resistivity measurements are used for, among other things, to infer the fluid content of pore spaces of the earth formations.
Induction electrical resistivity instruments known in the art, such as one described in Hunka, J. "A New Resistivity Measurement System for Deep Formation Imaging and High Resolution Formation Evaluation", paper no. 20559, Society of Petroleum Engineers, Richardson, Tex., 1990, typically measure voltages induced in receiver coils positioned at spaced apart locations along the instrument. The voltages are induced by magnetic fields generated by eddy currents flowing in the earth formations around the instrument, in response to alternating currents of various frequencies being passed through a transmitter coil. The voltages induced in a particular one of the receiver coils are dependent on the electrical conductivity (which is inversely related to resistivity) of the media, including the earth formations, surrounding the instrument, and are dependent on the spacing of the particular receiver coil with respect to the transmitter coil. Generally, the greater the spacing between the transmitter coil and the particular receiver coil, the greater is the radial depth from the wellbore into the earth formation from which the measurement at that receiver coil corresponds. The vertical resolution of the measurements made by the closer-spaced receiver coils, however, is proportionately finer than is the vertical resolution of the measurements made by the more distant receiver coils.
When the wellbore is drilled, fluids from the drilling process may be forced into the pore spaces of some of the earth formations, changing their fluid content and therefore their resistivity. The process of fluid being forced into the pore spaces is generally referred to as "invasion". Wellbore can be drilled with fluids having differing electrical resistivities, and the wellbore itself can have widely varying diameters over its length. The effects of variable wellbore diameter and differing wellbore fluid resistivity can indeterminately affect the total voltage being induced in each one of the receiver coils, because eddy currents also can flow within the conductive wellbore fluid and within that portion of the earth formations surrounding the wellbore subject to the invasion (the so-called "invaded zone").
U.S. Pat. No. 5,703,773 (referred hereafter as the "'773 patent") having the same assignee as the present application and the contents of which are fully incorporated herein by reference, discloses a method of inversion processing signals from an induction well logging instrument. The instrument includes a transmitter and a plurality of receivers at axially spaced apart locations. The method includes skin effect correcting the responses of the receivers by extrapolating the receiver responses to zero frequency. A model is generated of the media surrounding said instrument. Conductivities of elements in the model are adjusted so that a measure of misfit between the skin-effect corrected receiver responses and simulated receiver responses based on the model is minimized. The geometry of the model is adjusted so that the measure of misfit between the skin-effect corrected receiver responses and the simulated receiver responses based on the model is further minimized.
The method of the '773 patent and other prior art processing methods for well logs are developed for vertical or near vertical wells. If the well deviation exceeds 30.sup.0 -45.sup.0, traditional methods may fail. To interpret the data, 3-D modeling and inversion must be carried out. However, 3-D modeling and inversion is rarely practical due to the computational time requirements and the unavailability of powerful computers. As would be known to those versed in the art, some information about the subsurface is obtained using measurement-while-drilling ("MWD") methods in horizontal or near horizontal boreholes. In an MWD environment, there is relatively little invasion of the formation by the drilling mud and the relatively short transmitter to receiver spacings may be able to provide sufficient information about the formation resistivity to be useful in applications such as geosteering.
It is desirable to have an invention that is able to give information further away from the borehole in a near horizontal well. FIG. 1 is an end-on view of a borehole 10 within a reservoir rock 20. Surrounding the well is an invaded zone 12. Also shown in FIG. 1 are interfaces 22 and 26 marking a resistivity contrast between the zone 20 and zones 24 and 28. Typically, the interfaces 22 and 26 could be bed boundaries marking the separation between a reservoir formation 20 and non-reservoir formations 24 and 28. However, the boundary 26 could also be a fluid contact within a formation marking the separation between a water wet region 28 and a hydrocarbon wet region 20 while formation 22 is the caprock. Another possible configuration is that 20 is a water wet reservoir rock while 24 is the hydrocarbon saturated region with 28 being the seal beneath the reservoir.
As part of the reservoir evaluation process, it would be desirable to map the boundaries such as 22 and 26 accurately. Such information is useful in the estimation of reserves and in development of completion procedures for developing the reservoir. The present invention satisfies this need for situations such as in FIG. 1 and also in other situations where information relating to formation properties away from the borehole is needed.